Moulds for moulding a wind turbine blade are known in the art. Wind turbine blades can have a very large surface area, for example a length of up to 60 meters and a width of up to 5 meters. The blade surfaces are typically composed of fibre-reinforced resin matrix composite material which is moulded in the mould. The large mass of resin matrix material to be cured in a curing cycle means that heating of the mould is generally required to accelerate curing in order to control the curing cycle and ensure uniform and complete resin curing within a commercially acceptable curing cycle time.
It is known to provide tubes within the mould through which a heating fluid is passed to heat the mould surface during the curing cycle. However, such known mould incorporating heating systems are thermally inefficient, complex and expensive to manufacture.
In the past, it has been popular all over the world to heat wind turbine blade moulds by using water.
However this has the following disadvantages:    1. In winter time, the water can freeze, destroying the piping in the mould.    2. The water can cause corrosion in the pipes, destroying the mould.    3. Without pressure, heating over 90 C cannot be achieved.    4. If using pressurized water to achieve higher temperatures, dangerous leakage or explosion can occur. Also a safety inspection is needed for pressurized boilers.    5. If impure water is used, salt deposits can form inside the pipes, blocking them.
Effectively this has resulted in the limiting temperature for water boiler systems used for blade mould heating being set at 90 C. Since temperatures up to 80 C are needed for blade production, this causes a longer blade production cycle. The heating from 60 C up to 80 C becomes slower and slower as the blade mould temperature approaches the boiler temperature. This is because the effective heat transfer capability of the fluid drops further and further with the lessened temperature differential.
Further, an epoxy-fiberglass blade mould should be post-cured at a temperature of at least 110 C before use, in order to drive the curing of the mould shell to completion and get the most durable mould laminate. This cannot be achieved with a water heating system.
It has also been suggested to use silicone oil for heating, which would allow much higher boiler temperatures without pressure. However using such oil could cause silicone contamination inside the blade workshop. Silicone contamination would likely provoke failed gluing of the wind turbine blade, or defects in the blade painting. It is well known that any trace of silicone contamination is extremely harmful to adhesive bonding of all kinds.
So effectively until today, all blade moulds make use of electric heating, air heating, or water heating. In the short history of the wind blade production industry, no other heat transfer fluids have been discovered or used successfully.